The cytochromes P-450 metabolize a variety of drugs and carcinogens. The multiple forms of this enzyme display unique yet broad, substrate and reaction specificity. The focus of this project is the identification, characterization, and elucidation of structure-function relationships of these isozymes. Monoclonal antibodies (MAbs) to specific cytochromes P-450 are an essential tool in these studies. Several cytochromes P-450 have been substantially purified in a one-step immunoadsorption procedure using Sepharose bound MAbs to the major forms of rat liver cytochrome P-450 induced by 3-methylcholanthrene and phenobarbital (MC-P-450 and PB-P-450, respectively). When mixed with solubilized tissue microsomes, the immunoadsorbents bind polypeptides which are readily desorbed at pH 3.0. Cytochromes P-450 have been purified with MAbs 1-7-1 and 1-31-2- from livers of C57B1/6 and DBA/2 mice, guinea pigs and hamsters, and from rat lung. Such immunoadsorption experiments based on different MAbs reveal eptitope relatedness between sytochromes P-450 in different tissues, strains, and species. The cytochromes P-450 isolated by this procedure were analyzed structurally by peptide mapping and NH2-terminal amino acid analysis. Varying degrees of homology of these sytochromes P-450 were found. The hepatic cytochromes P-450 isolated from MC- and PB induced rats exhibit small yet detectable level of enzyme activity. A higher level of active cytochrome P-450 was prepared by a novel antigen-exchange method in which inactive denatured P-450 was exchanged for native cytochrome P-450 bound to the immunoadsorbents.